U.S. patent number 7,544,884 [Application Number 10/973,714] was granted by the patent office on 2009-06-09 for manufacturing method for large-scale production of thin-film solar cells.
This patent grant is currently assigned to Miasole. Invention is credited to Dennis R. Hollars.
United States Patent |
7,544,884 |
Hollars |
June 9, 2009 |
**Please see images for:
( Certificate of Correction ) ** |
Manufacturing method for large-scale production of thin-film solar
cells
Abstract
A method of manufacturing improved thin-film solar cells
entirely by sputtering includes a high efficiency back
contact/reflecting multi-layer containing at least one barrier
layer consisting of a transition metal nitride. A copper indium
gallium diselenide (Cu(In.sub.xGa.sub.1-x)Se.sub.2) absorber layer
(X ranging from 1 to approximately 0.7) is co-sputtered from
specially prepared electrically conductive targets using dual
cylindrical rotary magnetron technology. The band gap of the
absorber layer can be graded by varying the gallium content, and by
replacing the gallium partially or totally with aluminum.
Alternately the absorber layer is reactively sputtered from metal
alloy targets in the presence of hydrogen selenide gas. RF
sputtering is used to deposit a non-cadmium containing window layer
of ZnS. The top transparent electrode is reactively sputtered
aluminum doped ZnO. A unique modular vacuum roll-to-roll sputtering
machine is described. The machine is adapted to incorporate dual
cylindrical rotary magnetron technology to manufacture the improved
solar cell material in a single pass.
Inventors: |
Hollars; Dennis R. (San Jose,
CA) |
Assignee: |
Miasole (Santa Clara,
CA)
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Family
ID: |
32073349 |
Appl.
No.: |
10/973,714 |
Filed: |
October 25, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050109392 A1 |
May 26, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10671238 |
Sep 24, 2003 |
6974976 |
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60435814 |
Dec 19, 2002 |
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60415009 |
Sep 30, 2002 |
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Current U.S.
Class: |
136/256; 257/464;
257/433; 257/431; 257/43; 136/265; 136/264; 136/262; 136/252;
136/251; 136/245; 136/244 |
Current CPC
Class: |
C23C
14/3414 (20130101); C23C 14/0057 (20130101); C23C
14/352 (20130101); H01L 31/0322 (20130101); C23C
14/562 (20130101); H01L 31/022425 (20130101); H01L
31/0336 (20130101); H01L 31/18 (20130101); Y02P
70/50 (20151101); Y02E 10/541 (20130101); Y02P
70/521 (20151101) |
Current International
Class: |
H01L
31/04 (20060101) |
Field of
Search: |
;136/256,244,245,251,252,262,264,265 ;257/43,431,464,433 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Other References
R Manaila, D. Biro, A. Devenyi, D. Fratiloiu, R. Popescu, J. E.
Totolici, Structure of nitride film hard coatings prepared by
reactive magnetron sputtering, Applied Surface Sciencevol. 134,
Issues 1-4, , Sep. 1998, pp. 1-10. cited by examiner .
National Renewable Energy Laboratory, NREL/CP-520-22922--UC
Category 1250, Ullal, et al, entitled "Current Status of
Polycrystalline Thin-Film PV Technologies", Sep. 1997. cited by
other .
MRS Bulletin, Tadatsugu Minami, entitled "New n-Type Transparent
Conducting Oxides", pp. 38-44, Aug. 2000. cited by other.
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Primary Examiner: Neckel; Alexa D
Assistant Examiner: Mowla; Golam
Attorney, Agent or Firm: Foley & Lardner LLP
Parent Case Text
This application is a divisional of U.S. patent application Ser.
No. 10/671,238, filed on Sep. 24, 2003, now U.S. Pat. No. 6,974,976
which claims the benefit of U.S. Provisional Application No.
60/415,009, filed Sep. 30, 2002, and of U.S. Provisional
Application No. 60/435,814, filed Dec. 19, 2002.
Claims
What is claimed is:
1. A method of manufacturing a solar cell, comprising: passing a
web substrate from an input module to an output module through a
plurality of independently isolated, connected process modules at
the same time such that the web substrate continuously extends from
the input module to the output module while passing through the
plurality of the independently isolated, connected process modules;
depositing a conductive film on a surface of the substrate;
depositing at least one p-type semiconductor absorber layer on the
conductive film, wherein the p-type semiconductor absorber layer
includes a copper indium diselenide (CIS) based alloy material, and
wherein the deposition of the p-type semiconductor absorber layer
includes sputtering the CIS based alloy material from a pair of
conductive targets; depositing an n-type semiconductor layer on the
p-type semiconductor absorber layer to form a p-n junction; and
depositing a transparent electrically conductive top contact layer
on the n-type semiconductor layer; wherein each of the conductive
film, the at least one p-type semiconductor absorber layer, the
n-type semiconductor layer and the transparent electrically
conductive top contact layer are deposited at the same time on the
web substrate in a respective one of the plurality of the
independently isolated, connected process modules.
2. The method of claim 1, wherein the pair of conductive targets
comprises: a first target comprising a mixture of copper and
selenium; and a second target comprising a mixture of indium,
gallium, and selenium.
3. The method of claim 1, wherein the pair of conductive targets
comprises: a first target comprising a mixture of copper and
selenium; and a second target comprising a mixture of indium,
aluminum, and selenium.
4. The method of claim 1, wherein the pair of conductive targets
are disposed on dual cylindrical rotary magnetrons.
5. The method of claim 1, wherein the sputtering of the CIS based
alloy material further comprises: adjusting a power ratio between
the first target and the second target so that the deposited p-type
semiconductor absorber layer is slightly copper deficient.
6. The method of claim 1, wherein the sputtering of the CIS based
alloy material from the pair of conductive targets includes:
sputtering the CIS material from two or more pairs of the
conductive targets in a sequential manner, wherein the composition
of each pair of conductive targets varies relative to the other
pairs of conductive targets such that the deposited p-type
semiconductor absorber layer has a graded bandgap.
7. The method of claim 1, wherein the deposition of the n-type
semiconductor layer includes RF sputtering from a stoichiometric
zinc sulfide target in a magnetron configuration.
8. The method of claim 1, wherein the substrate comprises a thin
metallic foil.
9. The method of claim 8, wherein the thin metallic foil is
selected from stainless steel, copper, and aluminum.
10. The method of claim 1, wherein the CIS based alloy material is
selected from copper indium diselenide, copper indium gallium
diselenide and copper indium aluminum diselenide.
11. The method of claim 1, wherein the steps of depositing the
conductive film, depositing the at least one p-type semiconductor
absorber layer, depositing the n-type semiconductor layer, and
depositing the transparent electrically conductive top contact
layer are performed by sputtering.
12. The method of claim 11, wherein the steps of depositing the
conductive film, depositing the at least one p-type semiconductor
absorber layer, depositing the n-type semiconductor layer, and
depositing the transparent electrically conductive top contact
layer are performed by sputtering in a same sputtering
apparatus.
13. The method of claim 12, wherein the steps of depositing the
conductive film, depositing the at least one p-type semiconductor
absorber layer, depositing the n-type semiconductor layer, and
depositing the transparent electrically conductive top contact
layer comprise passing a metallic web substrate from the input
module to the output module though the plurality of process modules
in which each of the conductive film, the p-type absorber layer,
the n-type semiconductor layer and the transparent electrically
conductive top contact layer are deposited over the substrate by
sputtering.
14. A method of manufacturing a solar cell, comprising: passing a
metallic web substrate from an input module to an output module
through a plurality of independently isolated, connected process
modules at the same time such that the web substrate continuously
extends from the input module to the output module while passing
though the plurality of the independently isolated, connected
process modules; sputtering a conductive film on a surface of the
substrate in a first process module; sputtering at least one p-type
semiconductor absorber layer on the conductive film in a second
process module, wherein the p-type semiconductor absorber layer
includes a copper indium diselenide (CIS) based alloy material;
sputtering an n-type semiconductor layer on the p-type
semiconductor absorber in a third process module layer to form a
p-n junction; and sputtering a transparent electrically conductive
top contact layer on the n-type semiconductor layer in a fourth
process module; wherein each of the conductive film, the at least
one p-type semiconductor absorber layer, the n-type semiconductor
layer and the transparent electrically conductive top contact layer
are deposited at the same time on the web substrate in a respective
one of the plurality of the independently isolated, connected
process modules.
15. The method of claim 14, wherein the steps of sputtering the
conductive film, sputtering the at least one p-type semiconductor
absorber layer, sputtering the n-type semiconductor layer, and
sputtering the transparent electrically conductive top contact
layer are performed in a sputtering apparatus without breaking
16. The method of claim 14, wherein the CIS based alloy material is
selected from copper indium diselenide, copper indium gallium
diselenide and copper indium aluminum diselenide.
17. The method of claim 14, wherein the sputtering of the p-type
semiconductor absorber layer includes sputtering the CIS based
alloy material from a pair of conductive targets.
18. The method of claim 14, wherein the metallic web substrate
comprises a thin metallic foil web selected from stainless steel,
copper, and aluminum web.
19. The method of claim 14, further comprising cutting the web in
the output module.
20. The method of claim 14, wherein the step of sputtering at least
one p-type semiconductor absorber layer comprises reactively
sputtering the at least one p-type semiconductor absorber layer
from a pair of targets, each target comprising copper, indium and
gallium, in a selenium containing ambient.
21. The method of claim 20, wherein an atomic ratio of copper to
indium plus gallium in each target is less than one such that the
p-type semiconductor absorber layer comprises a copper deficient
copper indium gallium diselenide material.
22. The method of claim 21, wherein the selenium containing ambient
comprises hydrogen selenide gas provided into a sputtering
system.
23. The method of claim 1, further comprising cutting the web in
the output module.
24. The method of claim 1, wherein the step of sputtering the CIS
based alloy material from the pair of conductive targets comprises
reactively sputtering the CIS based alloy material from the pair of
targets, where each target comprises copper, indium and gallium, in
a selenium containing ambient.
25. The method of claim 24, wherein an atomic ratio of copper to
indium plus gallium in each target is less than one such that the
CIS based alloy material comprises a copper deficient copper indium
gallium diselenide material.
26. The method of claim 25, wherein the selenium containing ambient
comprises hydrogen selenide gas provided into a sputtering system.
Description
FIELD OF THE INVENTION
The invention disclosed herein relates generally to the field of
photovoltaics, and more specifically to a unique high throughput
roll-to-roll vacuum deposition system and method for the
manufacturing of thin-film solar cells based upon absorbing layers
that contain copper, indium, gallium, aluminum, and selenium and
have a polycrystalline chalcopyrite structure.
BACKGROUND OF THE INVENTION
Interest in thin-film photovoltaics has expanded in recent years.
This is due primarily to improvements in conversion efficiency of
cells made at the laboratory scale, and the anticipation that
manufacturing costs can be significantly reduced compared to the
older and more expensive crystalline and polycrystalline silicon
technology. The term "thin-film" is used to distinguish this type
of solar cell from the more common silicon based cell, which uses a
relatively thick silicon wafer. While single crystal silicon cells
still hold the record for conversion efficiency at over 20%,
thin-film cells have been produced which perform close to this
level. Therefore, performance of the thin-film cells is no longer
the major issue that limits their commercial use. The most
important factor now driving the commercialization of thin-film
solar cells is cost. Currently, a widely accepted technology
solution for the scale up to low-cost manufacturing does not
exist.
Attempts have been made and are now being made to remedy the
problem, but progress has been slow. While a large infrastructure
exists for the sputter coating of glass for the architectural
window market, this process is not readily adapted to the
production of solar cells for several reasons. First, the glass
that is coated in large-scale machines is relatively thick compared
to that used in solar modules. Also, the glass must be heated to
temperatures far above that required in the window industry,
causing large yield losses from fracturing and breakage. Handling
large sheets of glass is expensive in terms of floor space and
equipment, and the extra layers in a solar cell require additional
large coating chambers with appropriate gas isolation between
chambers. Finally, and maybe most importantly, efficient sputtering
targets have not yet been made for the deposition of the absorber
layer, which in many respects is the most challenging aspect of
making a thin-film solar cell.
An early attempt to improve manufacturing of solar cells with a
roll-to-roll technique was proposed by Barnett et al in U.S. Pat.
No. 4,318,938 ('938) issued 9 Mar. 1982. They describe a
roll-to-roll machine, which consists essentially of a series of
individual batch processing chambers each adapted to the formation
of a different layer. A thin foil substrate is continuously fed
from a roll in a linear belt-like fashion through the series of
individual chambers where it receives the required layers. Several
of the layers are formed by evaporation of the desired material in
vacuum chambers. The metal foil is transferred continuously from
air to vacuum and back to air several times. The patent does not
describe how this is accomplished, other than the statement that
such technology can be purchased. Much has changed in recent years.
The copper sulfide absorber layer proposed in '938 has been shown
to be unstable in the field, and some of the other layers are no
longer used. In particular, it is undesirable to have a pinch
roller running on a newly formed coating layer. However, the
inventors estimated that their continuous technique could reduce
the manufacturing cost by as much as a factor of two over the
conventional batch process for silicon. While a factor of two is
still significant today, greater reductions in cost must be
achieved if solar power is to become competitive with conventional
sources of power generation.
Matsuda et al in U.S. Pat. No. 5,571,749 ('749) issued 5 Nov. 1996
teach a roll-to-roll coating system based on plasma chemical vapor
deposition (CVD) techniques. Their system is a single linear vacuum
chamber with a series of six gas gates for process isolation. The
web substrate is passed through the machine in belt-like fashion
similar to the method of '938, but the web remains in vacuum for
the whole process. The solar cell absorbing layer is made from
amorphous silicon deposited from the decomposition of silane gas.
Different dopants are introduced along the belt path to create the
required p-n junctions. Similar techniques are used at Uni-Solar of
Troy, Mich. to make a variety of amorphous silicon solar cells. The
conversion efficiency of amorphous silicon cells is inferior to
that of the other thin film cells, and they suffer a loss of
efficiency during the initial few weeks of exposure to solar
radiation through a mechanism known as the Stabler-Wronski effect.
Because of this the efficiencies of amorphous silicon remain well
below that of other thin-film materials, and no one has yet found a
way to mitigate the effect.
Wendt et al disclose a roll-to-roll system in U.S. Pat. No.
6,372,538 ('538) issued 16 Apr. 2002 that teaches a method for
depositing a thin film solar cell based upon a copper
indium/gallium diselenide (CIGS) absorber layer. The system is
described as consisting of nine separate individual processing
chambers in which a roll-to-roll process may be used at each
chamber. Thus the overall system is similar to that described in
'938, but without the continuous belt-like transport of the
substrate through all of the chambers at once. Also the roll of
thin material (polyimide in this case) is not continuously fed
through a single vacuum system as it is in '749. Wendt et al teach
conventional planar magnetron sputtering for the deposition of a
molybdenum back contact layer onto the polyimide film. Adjustments
are made to the argon gas pressure and some oxygen is introduced to
adjust the film stress to accommodate the expansion of the
polyimide when it is heated for the CIGS deposition. Incorporation
of oxygen into the molybdenum layer increases its resistivity,
requiring the layer to be thicker to provide adequate electrical
conductivity. The CIGS materials are deposited over the molybdenum
layer in a separate chamber using an array of thermal evaporators
each depositing one of the components. The use of the polyimide
substrate material presents at least two problems in processing.
First, it contains a relatively large amount of adsorbed water,
which is evolved in the vacuum system and can have negative effects
on the process. And secondly, it cannot withstand the higher
temperatures used for the deposition of high quality CIGS material.
Thin foils of stainless steel would have neither of these problems.
The preferred width of the polyimide web is 33 cm, and it runs at a
typical line speed of 30 cm per minute. With respect to the present
invention, such production rates (about a square foot per minute)
are not considered large-scale; rather, rates 5 to 10 times faster
with attendant cost reductions are necessary to make solar power
competitive with power from conventional sources.
Copper indium diselenide (CuInSe.sub.2 or CIS) and its higher band
gap variants copper indium gallium diselenide (Cu(In/Ga)Se.sub.2 or
CIGS), copper indium aluminum diselenide (Cu(In/Al)Se.sub.2), and
any of these compounds with sulfur replacing some of the selenium
represent a group of materials that have desirable properties for
use as the absorber layer in thin-film solar cells. The acronyms
CIS and CIGS have been in common use in the literature for
sometime. The aluminum bearing variants have no common acronym as
yet, so CIGS is used here in an expanded sense to represent the
entire group of CIS based alloys. To function as a solar absorber
layer these materials must be p-type semiconductors. This is
accomplished by establishing a slight deficiency in copper, while
maintaining a chalcopyrite crystalline structure. Gallium usually
replaces 20% to 30% of the normal indium content to raise the band
gap; however, there are significant and useful variations outside
of this range. If gallium is replaced by aluminum, smaller amounts
of aluminum are required to achieve the same band gap.
CIGS thin-film solar cells are normally produced by first
depositing a molybdenum (moly) base electrical contact layer onto a
substrate such as glass, stainless steel foil, or other functional
substrate material. A relatively thick layer of CIGS is then
deposited on the moly layer by one of two widely used techniques.
In the precursor technique, the metals (Cu/In/Ga) are first
deposited onto the substrate using a physical vapor deposition
(PVD) process (i.e. evaporation or sputtering), chemical bath, or
electroplating process. Subsequently, a selenium bearing gas is
reacted with the metals layer in a diffusion furnace at
temperatures ranging up to about 600.degree. C. to form the final
CIGS composition. The most commonly used selenium bearing gas is
hydrogen selenide, which is extremely toxic to humans and requires
great care in its use. A second technique avoids the use of
hydrogen selenide gas by co-evaporating all of the CIGS
constituents onto a hot substrate from separate thermal evaporation
sources. While the deposition rates are relatively high for thermal
evaporation, the sources are difficult to control to achieve both
the required stoichiometry and thickness uniformity over large
areas of a substrate. Neither of these techniques for forming the
CIGS layer is readily scalable to efficient large-scale
production.
In part, moly is used as the back contact layer because of the high
temperature required for the CIGS deposition. Other metals (silver,
aluminum, copper etc.) tend to diffuse into and/or react with the
selenium in the CIGS at the elevated deposition temperatures, and
create an undesirable doping or interface between the contact layer
and the CIGS layer. Moly has a very high melting point (2610 C),
which helps to avoid this problem, although it will react with
selenium at high temperatures. However, even if the reactive
interface is minimized, moly still has a rather poor reflection at
the interface with the CIGS layer, resulting in decreased
efficiency since the light that penetrates the absorber initially
is not reflected back through the CIGS effectively for a second
chance at being absorbed. Therefore, replacing the moly with a
better reflecting layer can allow a decrease in the thickness of
the absorber layer as well as provide improved cell performance by
moving the absorption events closer to the p-n junction.
The n-type material most often used with CIGS absorbers to form the
thin "window" or "buffer" layer is cadmium sulfide (CdS). It is
much thinner than the CIGS layer and is usually applied by chemical
bath deposition (CBD). Cadmium is toxic and the chemical bath waste
poses an environmental disposal problem, adding to the expense of
manufacturing the cell. CBD zinc sulfide (ZnS) has been used
successfully as a substitute for CdS, and has produced cells of
comparable quality. The CBD method for ZnS is not as toxic as CdS;
but, remains a relatively expensive and time-consuming process
step, which should be avoided if possible. Radio frequency (RF)
sputter deposition of CdS and ZnS has been demonstrated on a small
scale. However, RF sputtering over large areas is difficult to
control because the plasma is highly influenced by the chamber
geometry in the conventional method of implementing RF sputtering.
An improved method of RF sputtering. ZnS is needed to reduce the
process complexity as well as to eliminate the toxic cadmium from
the process.
Finally, the window or buffer layer is covered with a relatively
thick transparent electrically conducting oxide, which is also an
n-type semiconductor. In the past zinc oxide (ZnO) has been used as
an alternative to the traditional, but more expensive, indium tin
oxide (ITO). Recently, aluminum doped ZnO has been shown to perform
about as well as ITO, and it has become the material of choice in
the industry. A thin "intrinsic" (meaning highly resistive) ZnO
layer is often deposited on top of the buffer layer to cover any
plating flaws in the CdS (hence "buffer" layer) before the cell is
completed by the deposition of the transparent top conductive
layer. In order to further optimize the performance of the cell, an
antireflection coating may be applied as a final step. Because of
differences in refractive index, this step is more important for
silicon cells than for CIGS cells in which some level of
antireflection is provided by the encapsulation material when the
cells are made into modules. In the case of CIGS an antireflection
coating may be applied to the outer surface of the glass.
The difficulties inherent in the deposition of CIGS related
absorber layers as well as the buffer layer have prevented these
thin-film solar cells from being readily manufactured in large
scale with improved economies and lower costs. Concurrent
improvements in the back reflector and elimination of cadmium and
its waste disposal problems can also lower the cost per watt of
generated solar power.
A conventional prior art CIGS solar cell structure is shown in FIG.
1. Because of the large range in the thickness of the different
layers, they are depicted schematically. The materials most often
used for each of the layers are also indicated in the figure. The
arrow at the top of the figure shows the direction of the solar
illumination on the cell. Element 1 is the substrate, and it is
massive in relation to the thin-film layers that are deposited upon
it. Glass is the substrate that has been commonly used in solar
cell research; however, it is more likely that for large-scale
production some type of foil-like substrate will be used. Layer 2
is the back electrical contact for the cell. Traditionally, it has
been moly with a thickness of about 0.5 to 1.0 microns. While moly
has been shown to be compatible with CIGS chemistry and the
relatively high temperature of the CIGS deposition, it has some
disadvantages. It is more expensive than other metals that are
better conductors (aluminum or copper for example), and it is not a
good reflector in the spectral region of the maximum solar output.
Thus light that does not create electron-hole pairs in the CIGS
absorber on its first transit is not efficiently reflected back
through the absorber for a second chance at causing a photoelectric
event. Light that is absorbed in the moly, including the part of
the solar spectrum that falls outside of the CIGS absorption band,
only contributes to heating of the cell, which lowers its overall
conversion efficiency. A better back electrode material is
desirable in a large-scale manufacturing system.
Layer 3 is the CIGS p-type semiconductor absorber layer. It is
usually about 2 to 3 microns thick, but could be somewhat thinner
and attain the same or improved efficiency if the reflection of the
back electrode layer (2) were improved. It would be extremely
desirable to produce this layer by magnetron sputtering. This would
enable a large-scale manufacturing process because magnetrons can
readily be made in large sizes, and thickness and composition
control can be excellent. A major provision of this invention is to
demonstrate how this can be done with CIGS materials. Layer 4 is
the n-type semiconductor layer that completes the formation of the
p-n junction. It is much thinner than the absorber layer (about
0.05 microns), and it should be highly transparent to the solar
radiation. Traditionally, it has been called the window layer,
since it lets the light pass down to the absorber layer. It is also
referred to as a buffer layer because it seems to help protect the
p-n junction from damage induced by the deposition of the next
layer. So far the use of CdS has resulted in the highest efficiency
cells for the CIGS type absorber materials. But CdS is
environmentally toxic, and it is difficult to deposit uniformly in
large-scale by either the chemical bath method or by conventional
RF magnetron sputtering. In addition, CdS is not highly transparent
to the green and blue region of the solar spectrum, which makes it
less compatible with higher band gap absorber layers.
At the 26.sup.th IEEE Photovoltaic Specialists Conference in
October of 1977 Ullal, Zweibel, and von Roedern suggested a list of
fifteen non-cadmium containing n-type materials that might be used
as substitutes for the CdS layer. Of those materials SnO.sub.2,
ZnO, ZrO.sub.2, and doped ZnO, are readily deposited by ordinary
reactive magnetron sputtering of the metal in an argon and oxygen
atmosphere. The reactive sputtering method that uses dual
cylindrical rotary magnetrons as taught in U.S. Pat. No. 6,365,010
('010) is especially useful for depositing these oxide layers.
However, the dual cylindrical rotary magnetron technology can
easily be extended to the reactive sputtering of sulfides and
selenides, if facility improvements are made to handle the delivery
of small amounts of the hydrogen sulfide and hydrogen selenide
gases to the reactive deposition region. Using this technique two
of the other materials on the list, ZnS and ZnSe, can be readily
deposited with the dual cylindrical rotary magnetron system in the
reactive mode. ZnS deposited by other methods has already been used
instead of CdS in a laboratory demonstration cell that achieved a
conversion efficiency of 18%. In addition, both ZnS and ZnSe have
larger band gaps than CdS, so they are more efficient window
materials. The less desirable method of conventional RF sputtering
would work marginally for depositing thin layers of any remaining
materials that cannot be readily formed into conducting
targets.
Layer 5 is the top transparent electrode, which completes the
functioning cell. This layer needs to be both highly conductive and
as transparent as possible to solar radiation. ZnO has been the
traditional material used with CIGS, but indium tin oxide (ITO), Al
doped ZnO, and a few other materials could perform as well. Layer 6
is the antireflection (AR) coating, which can allow a significant
amount of extra light into the cell. Depending on the intended use
of the cell, it might be deposited directly on the top conductor
(as illustrated), or on a separate cover glass, or both. For
space-based power it is desirable to eliminate the cover glass,
which adds significantly to expensive launch weight. Ideally, the
AR coating would reduce the reflection of the cell to very near
zero over the spectral region that photoelectric absorption occurs,
and at the same time increase the reflection in the other spectral
regions to reduce heating. Simple AR coatings do not adequately
cover the relatively broad spectral absorption region of a solar
cell, so multiple layer designs that are more expensive must be
used to do the job more efficiently. Coatings that both perform the
AR function and increase the reflection of unwanted radiation
require even more layers and significant coating system
sophistication. Aguilera et al in U.S. Pat. No. 6,107,564 issued 22
Aug. 2000 thoroughly review the prior art, and offer some improved
AR coating designs for solar cell covers.
As previously mentioned the moly back contact layer is not a good
reflector, nevertheless it has become the standard for thin-film
type solar cells. Finding a better reflecting material that would
otherwise withstand the processing conditions could improve the
cell performance. The task is not simple. The back layer
simultaneously should be a good conductor, be able to withstand
high processing temperatures, and it should be a good reflector.
Many metals in the periodic table meet at least one of these
requirements, and any metal could be made thick enough to provide
enough conductivity to function as the back electrical contact. The
requirement of high processing temperatures eliminates the low
melting point metals from consideration. Metals like tin, lead,
indium, zinc, bismuth, and a few others melt at temperatures below
the processing temperature for the CIGS or most other solar
absorber materials. The motivation to lower the cost of the cell
excludes metals like gold, platinum, palladium, rhodium, ruthenium,
iridium, and osmium which otherwise have good conduction and
reasonable reflection properties. With the exception of magnesium,
which is highly reactive, all of the rest of the metals on the left
half of the periodic chart of the elements are relatively poor
reflectors, including molybdenum. The remaining candidates include
aluminum, copper, silver, and nickel, and only nickel (and to a
lesser extent molybdenum) resists forming insulating and poorly
reflecting selenium compounds at the CIGS interface. However,
nickel will severely degrade CIGS material if it is allowed to
diffuse into it.
It is desirable to improve the large-scale manufacturability of
thin-film solar cells in order to reduce their cost and make them
competitive with conventional sources of electrical power
generation. The use of the term large-scale in the context of the
present invention implies the coating of either discrete substrates
or continuous webs that have width dimensions of about a meter or
more. This invention provides an apparatus and a method for sputter
depositing all of the layers in the solar cell, and particularly
the CIGS layer, which greatly increases the deposition area over
which the required properties of the material can be achieved and
controlled. It also provides improvements to the back
contact/reflecting layer and the elimination of cadmium from the
process.
SUMMARY OF THE INVENTION
An approach to solving problems with conventional CIGS solar cells
is presented by Iwasaki et al in U.S. Pat. No. 5,986,204 ('204).
They consider the same list of candidate metals that were just
discussed above; however, they propose using alloys of
silver-aluminum and copper-aluminum for the back conductor. A
limitation to use of these alloys is that they must be applied at
relatively low process temperatures, below about 120 C., which are
marginally appropriate for amorphous silicon absorber layers, but
would not work for CIGS at its normal processing temperatures. In
addition the patent teaches the use of a clear conducting oxide
(ZnO) as a barrier layer between the alloy and the absorber layer,
as well as placing the alloy on a textured base metal layer to
increase the scattering angle. The ZnO layer provides conductivity
and inhibits migration, but like all useful clear conducting
oxides, it is an n-type semiconductor. When it is placed against
the p-type absorber layer, a weak p-n junction is formed, which
acts to apply an undesirable small reverse electrical bias to the
cell. The primary p-n junction must then overcome this back bias to
cause useful current to flow, thus degrading the net
efficiency.
Iwasaki et al are on the right track, but there are two impediments
to the performance of their reflector. First, the ZnO barrier layer
should not be an n-type semiconductor; and secondly, alloys
generally have poorer conductivity and reflectivity than the pure
metals. Among the transition metal nitrides, borides, silicides,
and carbides, several have high electrical conductivity; and
additionally, they have high melting temperatures and are
relatively inert. A few have desirable optical properties. The most
optimum materials are the nitrides of some of the transition
metals, and in particular titanium nitride (TiN), zirconium nitride
(ZrN), and hafnium nitride (HfN). These nitrides have high melting
points (about 3000 C. for ZrN) and higher electrical conductivity
than their parent metals; therefore, they do not act as
semiconductors. Additionally, they have good optical properties;
specifically, low indices of refraction similar to the noble
metals. These properties make them very useful for forming an
improved back contact/reflecting layer in solar cells. All of the
above mentioned nitrides work well, but zirconium nitride has
somewhat better optical and electrical properties, and it is the
one discussed herein as representative of the entire class of metal
nitrides.
FIG. 2 shows the computed reflectivity in air of 0.5 micron thick
(opaque) films of molybdenum, niobium, nickel, copper, silver,
aluminum, and zirconium nitride from 400 to 1200 nm. This spectral
range covers the principal region of the solar radiant output,
which lies above a photon energy of about 1 electron volt (ev). For
a single junction solar cell a band gap of 1.4 to 1.5 ev is the
optimum for highest efficiency, and in this region niobium and
molybdenum, have reflection that is inferior to that of any of the
other metals. The metallic nature of zirconium nitride is evidenced
by its relatively high reflectivity as compared to molybdenum,
niobium, and nickel. The reflectivity of a metal in air depends
upon the optical indices of the air and the metal, which of course
vary with wavelength. The simple formula for the reflection at the
air/metal interface is:
##EQU00001## where n.sub.o is the refractive index of air
(.about.1) and n.sub.m and k.sub.m are the refractive index and
extinction coefficient of the metal. For a metal like silver the
refractive index is much smaller than one, and the extinction
coefficient is larger than one, so the k.sub.m.sup.2 term dominates
and the reflection approaches 100% for thick films. In the case of
molybdenum, niobium, and nickel both n and k are greater than one
in the visible spectral region, so their reflections work out to be
substantially less because of the (n.sub.m-\+n.sub.o).sub.2
terms.
It happens that most semiconductors also have a refractive index of
about 3, and this is particularly true for CIGS and CdTe, two of
the leading contenders for thin-film solar cell absorbers. The
formula for the reflection suggests that the back reflection layer
should not have n and k values near to 3. It seems that few if any
in the industry have noticed or discussed this potential problem
with molybdenum in particular. FIG. 3 shows the computed reflection
of these metals at the interface between the CIGS layer and the
metal back conducting and reflecting layer, which is the way the
layer actually functions in the solar cell. Note that, as suggested
above, the reflection of the molybdenum is greatly reduced from its
value in air--by more than a factor of 2 in the most critical
spectral regions. The reflections of niobium and nickel fair
somewhat better, but are also reduced significantly. The
reflections of the other metals are not reduced as much because
their refractive indices differ more markedly from the value 3.
Nickel is a better reflector than molybdenum, and it would be more
economical; however, its tendency to diffuse is a potential
problem, and since it is magnetic, it is more difficult to sputter
than non-magnetic metals. Zirconium nitride would be an excellent
solution, with much better reflection than molybdenum, niobium, or
nickel. However, the zirconium nitride would need to be about 1.5
microns thick to provide the same total electrical conductivity as
0.5 microns of molybdenum. It is possible to manufacture such a
thick film with some economy using reactive sputtering; however,
there is a better solution.
FIG. 4 shows the reflection of the metals in the previous two
figures when a 15 nm thick barrier layer of zirconium nitride is
placed between the CIGS layer (or a CdTe layer) and the metal
layer. The reflection of the molybdenum, niobium, and nickel are
significantly improved, while the reflections of the other metals
are slightly reduced. As the thickness of the zirconium nitride
layer is further increased, the reflectance at the interface for
all of the metals approaches that of the thick zirconium nitride
shown in FIG. 7 (over 70%). In fact calculations predict that, at a
zirconium nitride barrier layer thickness of approximately 100 nm
(or 0.10 microns), the metal layer underneath the zirconium nitride
has little to no effect on the reflectance of light back through
the CIGS layer--it becomes totally dominated by the properties of
the zirconium nitride barrier layer.
As an example FIG. 5 shows the reflection at a wavelength of 800 nm
for molybdenum and silver at the absorber/reflector interface as
the thickness of the zirconium nitride layer varies from zero to
200 nm. For molybdenum the reflection first increases sharply as
the thickness of the zirconium nitride barrier layer increases, but
it begins to roll off at a thickness of about 30 nm and changes
very slowly after a thickness of about 60 nm. At a thickness of 100
nm further change in the reflection is imperceptible. Reflection
results for niobium and nickel (not shown) behave in a manner very
similar to molybdenum. The reflections start at a higher level than
molybdenum, but they quickly approach the same limit. For silver
the reflection starts out at high reflection (about 95%) and falls
to that of thick zirconium nitride over about the same thickness
range as the case for molybdenum. In general metals that are poor
reflectors need thicker zirconium nitride barrier layers, and
metals that are very good reflectors should have thinner barrier
layers, i.e. just enough to do the job of protecting the
absorber/reflector interface. So a thin layer of ZrN acts like a
metal and prevents the formation of an inverse p-n junction. It
improves the reflection of the optically poor metals, and protects
the CIGS layer from diffusion by the highly reflective metals.
Since the optical properties are separated from the conductivity
requirements of the back contact layer, a wider range of choices
for the base metal layer are possible.
Accordingly, the present invention relates to a roll-to-roll
deposition apparatus and method for producing an all sputtered
thin-film CIGS solar cell, wherein the CIGS absorber layer is
formed by co-deposition from a pair of rectangular planar or
cylindrical rotary magnetrons using direct current (DC) sputtering.
The buffer layer of ZnS is RF sputtered from a conventional planar
magnetron housed in a special chamber, thus replacing the toxic CdS
with a more benign material. The remaining layers in the cell are
formed by deposition from dual magnetrons utilizing DC and
alternating current (AC) sputtering. Thus the cell is manufactured
in a single pass through a large modular vacuum deposition machine
with no wet processes or high temperature gas diffusion processes
involved. The back contact/reflecting layer is improved by the
addition of a material not used in solar cells previously. In a
preferred-embodiment of the invention, the CIGS layer is deposited
from dual cylindrical rotary magnetrons, used in the configuration
that is described in U.S. Pat. No. 6,365,010 (which is incorporated
herein by reference), in which one target contains copper and
selenium while the second target contains indium, gallium, and
selenium or indium, aluminum, and selenium.
A primary objective of the present invention is to provide a
large-scale manufacturing system for the economical production of
thin-film CIGS solar cells.
An additional objective of the invention is to provide a
manufacturing protocol for solar cells in which high temperature
toxic gases and toxic wet chemical baths are eliminated from the
process.
Another objective of the invention is to provide a manufacturing
process for CIGS solar cells, which significantly lowers their
cost, specifically by improvements in the back contact/reflection
layer and the elimination of cadmium and the disposal of its toxic
wastes.
A further objective of the invention is to provide an apparatus and
manufacturing process for CIGS solar cells, which significantly
increases the size of substrate that can be used, including
primarily continuous webs of material deposited in a special
customized and modularized roll-to-roll coating machine with
increased capabilities and efficiencies.
The present invention is a method of manufacturing a solar cell
that includes providing a substrate, depositing a conductive film
on a surface of the substrate wherein the conductive film includes
a plurality of discrete layers of conductive materials, depositing
at least one p-type semiconductor absorber layer on the conductive
film, wherein the p-type semiconductor absorber layer includes a
copper indium diselenide (CIS) based alloy material, depositing an
n-type semiconductor layer on the p-type semiconductor absorber
layer to form a p-n junction, and depositing a transparent
electrically conductive top contact layer on the n-type
semiconductor layer.
In another aspect of the present invention, a method of
manufacturing a solar cell includes providing a substrate,
depositing a conductive film on a surface of the substrate,
depositing at least one p-type semiconductor absorber layer on the
conductive film wherein the p-type semiconductor absorber layer
includes a copper indium diselenide (CIS) based alloy material, and
wherein the deposition of the p-type semiconductor absorber layer
includes co-sputtering the CIS material from a pair of conductive
targets, depositing an n-type semiconductor layer on the p-type
semiconductor absorber layer to form a p-n junction, and depositing
a transparent electrically conductive top contact layer on the
n-type semiconductor layer.
In yet another aspect of the present invention, a method of
manufacturing a solar cell includes providing a substrate,
depositing a conductive film on a surface of the substrate,
depositing at least one p-type semiconductor absorber layer on the
conductive film wherein the p-type semiconductor absorber layer
includes a copper indium diselenide (CIS) based alloy material and
wherein the deposition of the p-type semiconductor absorber layer
includes reactively AC sputtering material from a pair of identical
conductive targets in a sputtering atmosphere comprising argon gas
and hydrogen selenide gas, depositing an n-type semiconductor layer
on the p-type semiconductor absorber layer to form a p-n junction,
and depositing a transparent electrically conductive top contact
layer on the n-type semiconductor layer.
In yet one more aspect of the present invention, a solar cell
includes a substrate, a conductive film disposed on a surface of
the substrate wherein the conductive film includes a plurality of
discrete layers of conductive materials, at least one p-type
semiconductor absorber layer disposed on the conductive film
wherein the p-type semiconductor absorber layer includes a copper
indium diselenide (CIS) based alloy material, an n-type
semiconductor layer disposed on the p-type semiconductor absorber
layer wherein the p-type semiconductor absorber layer and the
n-type semiconductor layer form a p-n junction, and a transparent
electrically conductive top contact layer on the n-type
semiconductor layer.
In still yet one more aspect of the present invention, a vacuum
sputtering apparatus includes an input module for paying out
substrate material from a roll of the substrate material, at least
one process module for receiving the substrate material from the
input module, and an output module. The process module includes a
rotatable coating drum around which the substrate material extends,
a heater array for heating the coating drum, and one or more
sputtering magnetrons each having a magnetron housing and a
plurality of conductive sputtering targets disposed in the
magnetron housing and facing the coating drum for sputtering
material onto the substrate material. The output module receives
the substrate material from the process module.
Other objects and features of the present invention will become
apparent by a review of the specification, claims and appended
figures.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration of the prior art structure of a
basic CIGS solar cell.
FIG. 2 shows the computed reflection in air of metals commonly
considered useful as solar cell back contact layers. Included is a
new class of materials represented by zirconium nitride.
FIG. 3 shows the computed internal reflection at the interface
between a CIGS absorber layer and the metals and zirconium nitride
shown in FIG. 2.
FIG. 4 shows the computed internal reflection at the interface
between a CIGS absorber layer and the metals shown in FIG. 2 with a
15 nm thick layer of zirconium nitride placed at the interface.
FIG. 5 shows the reflection at 800 nm at the absorber/reflector
interface in a solar cell as a function of the thickness of a
zirconium nitride barrier layer.
FIG. 6 shows the structure of the basic solar cell of the present
invention, wherein a thin zirconium nitride is inserted between the
CIGS layer and the back conducting/reflecting metal layer.
FIG. 7 shows an alternative structure for the solar cell of the
present invention, wherein the back conducting/reflecting layer is
improved with copper and silver layers.
FIG. 8 shows schematically the co-sputtering of CIGS material from
conventional dual rectangular planar magnetrons.
FIG. 9 illustrates schematically the preferred embodiment of the DC
co-sputtering of CIGS material from dual cylindrical rotary
magnetrons.
FIG. 10 shows schematically an alternative method of using AC power
to co-sputter the CIGS material.
FIG. 11 shows schematically an alternative AC reactive sputtering
method using dual cylindrical rotary magnetrons with identical
metal alloy targets to form the CIGS material.
FIG. 12 illustrates schematically the use of three sets of dual
magnetrons to increase the deposition rate and grade the
composition of the CIGS layer to vary its band gap.
FIG. 13 shows the structure of a preferred embodiment of an
improved all sputtered version of the basic solar cell of the
present invention.
FIG. 14 shows a highly simplified schematic diagram of the side
view of a roll-to-roll modular sputtering machine used to
manufacture the solar cell depicted in FIG. 13.
FIG. 15 shows a more detailed schematic diagram of a section of a
process module with details of the construction of the coating drum
and the magnetron.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention will now be described and compared with respect to
the conventional prior art CIGS solar cell structure. The new cell
structure and the manufacturing process will be detailed in
relation to a modular roll-to-roll sputter deposition system
designed specifically to provide an optimum implementation of the
process.
It should be noted that, as used herein, the terms "over" and "on"
both inclusively include "directly on" (no intermediate materials,
elements or space disposed therebetween) and "indirectly on"
(intermediate materials, elements or space disposed therebetween).
For example, forming an element "on a substrate" can include
forming the element directly on the substrate with no intermediate
materials/elements therebetween, as well as forming the element
indirectly on the substrate with one or more intermediate
materials/elements therebetween.
FIG. 6 illustrates one of the simplest embodiments of a basic solar
cell according to the present invention, which includes a zirconium
nitride barrier layer. The figure is similar to the conventional
solar cell shown in FIG. 1 except for the added barrier layer 2a of
zirconium nitride between the CIGS layer 3 and electrical contact
layer 2. As suggested above, electrical contact layer 2 can now be
any of the metals that were discussed above or any economical metal
with adequate conductivity. The alloys claimed by Iwasaki et al in
'204 will work since the zirconium nitride barrier layer will block
diffusion while retaining good reflectivity. Pure silver would give
the optimum in performance; however, it would be a relatively
expensive solution. Aluminum is the cheapest good reflector, but
its melting point is relatively low (660 C.) compared to the other
metals, and it getters oxygen from the background water vapor in a
vacuum system, which lowers its conductivity.
An alternate embodiment of the solar cell of the present invention
is shown in FIG. 7, where electrical contact layer 2 is made from
copper instead of molybdenum. Copper is relatively inexpensive and
a very good conductor. At about 0.2 microns thickness, it provides
as much electrical conductivity as 0.5 microns of molybdenum. Layer
2a is a thin barrier layer of zirconium nitride that has a
thickness in the range of approximately 10 to 20 nm. At this point
the layer structure is the same as that discussed in FIG. 6, and it
could be used in this form, especially with CIGS because of its
modest band gap. However, the lower reflection shortward of about
600 nm (see FIG. 4) can be remedied by a thin layer 2b of silver
(40 to 50 nm) deposited on top of the zirconium nitride layer, and
an additional barrier layer 2c of zirconium nitride between the
silver and the CIGS layers. With this structure the internal
reflection at the silver/ZrN/CIGS interface is practically
indistinguishable from the reflection curve labeled "silver" in
FIG. 4, and the use of large amounts of the more expensive silver
is avoided. If the CIGS processing temperature could be
significantly lowered from the current value of about 550 C., then
the intermediate barrier layer 2a between the copper and silver
could be eliminated for temperatures below which copper and silver
would rapidly inter-diffuse. Alternatively, for a low enough
processing temperature aluminum could be substituted for both the
copper and the silver. Of course, if the substrate were a metal
foil instead of glass, the base metal layer could be made thinner
while retaining the necessary reflectivity, since the metal foil
would provide most of the conductivity.
The next layer to be described is the CIGS absorber. In this
invention the preferred deposition method for the CIGS material is
DC magnetron sputtering; however, AC reactive magnetron sputtering
is also a viable alternative method diminished only by the added
necessity of handling small amounts of the toxic gas hydrogen
selenide. Both methods utilize the magnetron technology taught in
'010 as the most desirable method; although, the invention may be
practiced less effectively with conventional planar magnetrons. One
reason DC magnetron sputtering has not been done with the CIGS
material is that the electrical conductivity is too low because of
its semiconductor nature. DC sputtering requires metal-like
electrical conductivity as well as good thermal conductivity to
allow high power for high deposition rates. Conceptually, one
important idea in this invention is to divide the CIGS material
into two parts, each part having properties that would permit the
fabrication of a conductive sputtering target. For the majority of
semiconductors that are serious candidates for use as absorber
layers in solar cells this would be impossible, but the results of
recent experiments demonstrate that it works for CIGS. After
several failed attempts of various combinations, it was discovered
that copper and selenium could be combined into a conductive matrix
provided that the material was processed properly. A
homogeneous-mixture of powders consisting of approximately two
parts Se and one part Cu remains highly conductive if it is cold
pressed and annealed at a temperature somewhat below the melting
point of Se (217.degree. C.). Small samples made at 208 to
210.degree. C. had good physical strength, and the electrical
resistance was less than one ohm. When the annealing temperature is
raised to about 400 C. the resistance goes up by a factor of more
than a million as might be expected from the formation of
CuSe.sub.2. But the conductive properties of the lower temperature
material are not easily reconciled with the existing phase diagrams
for the Cu--Se binary system. If chemical reactions between the Cu
and Se do not occur at the low annealing temperature then the Se
could be acting as a binder to hold a highly conductive copper
matrix together. For this to be the case, the Cu would have to
diffuse rapidly at the low annealing temperature, which is
unlikely. The Cu.sub.2Se phase is the only Cu/Se phase known to be
conductive, so it probably forms although it does not appear to be
consistent with the phase diagram for this composition and
temperature. However, since the material changes its appearance
after annealing, it seems to favor a reaction having occurred.
Similar experiments in which the Cu was replaced with In did not
yield highly conductive matrices. In fact the resistance increased
with indium content even at low annealing temperatures. Since In
and Se have low melting points, the observed result might be
expected, and unlike the Cu, it is consistent with the In--Se phase
diagram.
Regardless of the inconsistency with the phase diagram, the Cu/Se
has been made with the necessary properties for high rate DC
magnetron sputtering. The target for the rest of the material must
contain the indium and gallium needed to complete the CIGS
structure. In and Ga are readily melted together to form a low
temperature solder which can be poured or cast into a mold
surrounding a backing or carrier tube to form the target. Good
mixing and rapid quenching is required to prevent segregation and
formation of the low temperature eutectic. A more desirable
approach is to form the target by compressing metal powders, and in
particular to include the gallium as gallium selenide
(Ga.sub.2Se.sub.3). The target remains conductive and the low
temperature eutectic is avoided. Additional Se may also be added
and reacted with the In to form the insulating In.sub.2Se.sub.3
phase, but as long as enough free In is left to form a conductive
matrix, the target will sputter adequately. About half of the In/Ga
target can be Se, and remain conductive enough to sputter since it
takes three atoms of Se for every two atoms of In or Ga. Replacing
the gallium with aluminum raises the eutectic melting point
substantially, without causing any further technical difficulties.
The inclusion of selenium in the In/Ga or In/Al target in addition
to the selenium from the copper target provides an overpressure of
selenium during the deposition process that is highly
desirable.
Another advantage this target construction technology offers is a
way to dope the materials in many different and potentially
beneficial ways. For example, it has been known for a long time
that a very small amount of sodium (Na) added to the CIGS can
improve its performance. Initially, it was-noticed that cells made
on soda lime glass had higher efficiency than those made on other
substrates, particularly stainless steel. Later it was discovered
that traces of Na from the glass were diffusing into the CIGS
during deposition. However, a way to add a small but controlled
amount of Na easily for non-glass substrates has proved difficult.
With the target forming method of this invention it is easy to
introduce trace amounts (e.g. about 0.1%) of NaSe.sub.2 into either
the Cu/Se or the In/Ga/Se to achieve the desired doping in the
absorber layer.
The following description of sputtering CIGS material is made with
respect to a pair of sputtering targets: one comprising of Cu and
Se, and the other In, Ga, and Se. The ratio of Cu to Se is
approximately 1 to 2, but may be varied to accommodate process
variations and requirements. The In to Ga ratio is varied to change
the band gap, and it can range from In alone (band gap of 1 ev) to
about 30% Ga (band gap of 1.3 ev). It should be noted that changes
in the ratios of the materials in each target as well as the
additions of small levels of doping (as described above for Na)
with other elements are considered to be consistent with the basic
invention.
Conventional DC rectangular planar magnetron co-sputtering of the
CIGS material is shown schematically in FIG. 8. The view is a
cross-section taken perpendicular to the long axes of the
magnetrons. Elements 7 represent the main bodies of the
conventional magnetrons, which house the magnetic assemblies (not
shown) that form the sputtering "racetrack" and the means of
cooling targets 8 and 9. The magnetrons are oriented so that a line
perpendicular to each target intersects at substrate 10, which is
approximately 10 cm away. Each magnetron is powered by a DC power
supply 11, which is grounded to chamber wall/shield 12 that serves
as the system anode. Alternatively, separate anodes (not shown)
intimately associated with each magnetron could be provided as is
common in the art. Baffle 13 is placed between the magnetrons to
help restrict material sputtered from one source from depositing
on, and reacting with, that of the other source. The reacted
material would be largely insulating and therefore undesirable
since it builds up over time on areas of the planar target that are
not sputtering. The baffle should not protrude toward the substrate
so far that the flux reaching the substrate is significantly
reduced. If grounded, as indicated, it could function as an anode
or partial anode for each magnetron. All sputtering processes use a
working gas, which is almost universally argon. Since it is inert,
it may be introduced almost anywhere in the system. In FIGS. 8
through 12 the argon injection location is not shown explicitly;
however, injection at the rear or sides of the magnetron is
conventional and appropriate.
Still referring to FIG. 8, one of the targets, 8 for instance,
comprises the conductive Cu/Se material, while target 9 comprises
the conductive In/Ga/Se material. A substrate 10 is heated to a
temperature of between 400 and 600 C., and is transported past the
magnetrons at a uniform rate as indicated by the arrow. Argon is
introduced as the working gas at a pressure of approximately 1 to 2
millitorr, and DC power is applied to sputter the materials. One
power supply (11) is adjusted to achieve an acceptable sputtering
rate for one of the two targets. The other power supply is then
adjusted until the reacted coating on the heated substrate has the
correct copper deficient composition. If the several constituents
in each target have the same sputtering distribution pattern
(although the two target patterns may differ from each other) then
the correct composition will be achieved with power supply
adjustments alone. In general this may not be the case, in part
because the individual elements may have different sputtering
patterns. Thus, one constituent may be preferentially collected on
nearby shielding, shifting the coating composition slightly from
that expected from the original target composition. Adjustments in
the compositions of each target by only few percent will correct
the discrepancy, but the exact composition is dependent on many
factors including machine geometry, sputtering pressure and
sputtering power, so the correct compositions must be worked out
for each unique machine setup. Once the compositions are
determined, they remain constant until there is a change in the
process or system geometry. Such small variations in target
composition to accommodate system geometry are considered to fall
within the scope of this invention.
Process problems develop with the rectangular planar magnetron
embodiment when the targets are sputtered for long periods of time,
as would be the case for a large-scale manufacturing operation. A
sputtering groove 14 (dashed line) gradually forms defining the
"racetrack" on each target as the deposition process proceeds. The
well-known cosine distribution, which describes the local flux
emission pattern, is oriented perpendicular to the emitting
surface. Therefore, the flux distribution at the substrate
gradually changes as the target erodes and the groove forms. If the
patterns from the two magnetrons do not change in synchronism with
each other, the composition of the CIGS material deposited at the
substrate will change with time, requiring adjustments to be
determined and applied to the process almost continuously.
A second problem is that baffle 13 will not totally stop flux
mixing between the targets over extended run times. This means that
eventually significant amounts of partially insulating reaction
products will build up on regions of the targets that are not being
sputtered (i.e. at the edges of the "racetrack"). This can lead to
arcing and defects in the CIGS film. Finally, the utilization of
the target material ranges from about 25 to 40% for the planar
targets, and they must be changed often, thus raising the
manufacturing costs.
If cylindrical rotary magnetrons are substituted for the planar
magnetrons in FIG. 8, the setup becomes that shown in FIG. 9, where
common and similar elements are labeled with the same numerals. If
they are operated identically to that of the planar magnetron
setup, the problems associated with the planar magnetron embodiment
are largely eliminated. Since they rotate, a sputtering groove
never forms. So the composition of the coating remains constant as
the target material is consumed, because the emission pattern of
the flux remains fixed. Also, because of the rotation and
subsequent continual target cleaning, there can be no long term
increasing build up of reacted material on the targets for the same
reasons that pertain to reactive sputtering as detailed in '010.
For this reason baffle 13 (shown dashed) is not as important in the
rotary magnetron embodiment. If the diameter of the rotary target
is equal to the planar target width, and the target material is the
same thickness, then the rotary target has over three times the
initial inventory of material as the planar. And, because the
utilization is more than double that of the planar, the rotary
targets will run more than six times as long as the planar-targets
before target changes are necessary. This is a significant cost
saving factor for large-scale manufacturing.
Either the planar or the rotary magnetrons could be run in AC mode.
This is illustrated in FIG. 10 for the rotary magnetrons, but the
setup would apply as well to the planar magnetrons. The two DC
power supplies 11 are replaced by a single AC power supply 15. In
order to vary the deposition rates between the targets to maintain
a copper deficient film composition, a variable impedance load 16
must be inserted into one of the legs of the AC supply. Since AC
operation with dual magnetrons does not require a separate anode,
the chamber wall/shields 12 no longer need to be grounded and
neither does baffle 13. This alternative setup using AC power when
the conductive targets will support DC operation offers little
advantage for the rotary magnetrons, but since the planar
magnetrons are not self-cleaning, it could offer some protection
from arcing in that setup.
As mentioned above AC reactive sputtering of the CIGS material is a
viable alternative to DC sputtering if facility arrangements are
made to handle small amounts of hydrogen selenide gas, or other
potentially useful gases. FIG. 11 shows this setup for a pair of
rotary magnetrons. It differs from that shown in FIG. 10 in a
number of respects.
First, targets 8 and 9 are now identical, consisting of an alloy of
the metals copper, indium, and gallium (or aluminum) selected to
give a slightly copper deficient composition and a desired band
gap. Basically the atomic ratio of copper to indium plus gallium or
aluminum should be slightly less than one, with the ratio of indium
to gallium or aluminum determining the band gap. The metal targets
can be made using conventional melting and casting techniques.
Since the targets are now identical in composition, baffle 13 may
also be eliminated. In addition to using argon as the conventional
sputtering gas, hydrogen selenide gas is fed into the system near
the substrate through, for example, nozzles 17 to react with the
sputtered metal atoms and form the CIGS material in a continuous
process.
To date the best candidates for high efficiency thin-film solar
absorbers contain materials that must be made in complex
structures, or that form or use compounds and gases that are toxic.
At the present time there is one class of materials that show some
promise for changing this situation, and those are the nitrides of
the IIIA elements aluminum, gallium, and indium. The nitrides of
various mixtures of In/Ga and In/Al display a range of band gaps
that span the range of the solar spectrum. So far the techniques
for making them as p-type semiconductors have not been perfected.
Such an absorber system would be ideal for production with the
rotary magnetrons of '010. The setup would be like that shown in
FIG. 11 except the toxic reactive gas hydrogen selenide would be
replaced with harmless nitrogen. The transition metal nitride layer
(i.e. ZrN) previously discussed is formed precisely in this
way.
Since the CIGS layer (or other absorber layer) is relatively thick,
the throughput of the sputtering machine can be improved by using
two or more pairs of magnetrons to deposit the layer. Because the
cost of the magnetrons is moderate compared to the overall cost of
the vacuum system, the increased production rates more than offset
the moderate increase in the initial capital costs. For an in-line
machine that coats discrete substrates, throughput can also be
increased by placing magnetron sources on both sides of the
machine, and coating two substrates on the same pass. The need to
use multiple pairs of magnetrons to increase the rate of deposition
of the CIGS layer presents another opportunity that is exploited in
the present invention. This is discussed below using a
representative example.
FIG. 12 illustrates schematically the CIGS deposition region within
a sputtering machine equipped with three pairs of rotary (shown) or
planar (not shown) magnetrons. It could represent a region from an
in-line machine, or if arranged in an arc, a region from a
roll-to-roll coater with a web substrate carried on a drum. With
respect to the direction of motion of substrate 10 (indicated by
the arrow), the first pair of magnetrons is 18, the second 19, and
the third 20. In each pair of magnetrons, one of the targets is
Cu/Se with a properly adjusted composition as discussed above.
However, the second target in each group would be, for example,
just In/Se for 18, In/Se with 15% Ga for 19, and In/Se with 30% Ga
for 20. In this way the Ga content of the CIGS layer would be
step-wise graded from bottom to top with little or no Ga in the
bottom region and some maximum amount of Ga in the top region. This
will grade the band gap from about 1 ev at the bottom to about 1.3
ev near the top of the layer. Inverting the target sequence or
coating in the reverse direction would invert the band gap grading.
Some smoothing of the stepped boundary could be obtained by placing
the magnetrons close enough together to allow some overlap in their
deposition patterns; however, thermal diffusion of the material
will cause some grading at the interface between regions in any
event.
The advantage of being able to adjust the CIGS composition easily
by using multiple sets of targets is that the band gap of the CIGS
can be engineered to optimize the efficiency of the cell.
Conventional wisdom would suggest forming the highest band gap
regions at the top layer and the lowest band gap regions at the
bottom in the same order as that used in multi-junction cells.
However, in practice inverting this structure in a single junction
cell generally leads to improved efficiency through a broadening of
the voltage gradient across the absorber. Without Ga (or Al) in the
CIGS the band gap is about 1 electron volt (ev), while the optimum
for the solar spectrum is about 1.4 to 1.5 ev. Replacing In with
30% Ga raises the band gap to about 1.2 ev. Further additions of Ga
start to lower cell efficiency. If Ga totally replaces In, the band
gap can exceed 1.6 ev. Aluminum raises the band gap faster than Ga,
allowing a bandgap of 1.45 without exceeding 30%. Sulfur
replacement of some of the selenium also raises the band gap, but
less effectively than Ga. Many combinations are possible, and for a
wide range of additive materials the targets, when fabricated as
described herein, remain conductive enough to co-sputter by DC
methods. If p-type nitrides can be perfected, the magnetron targets
would be fabricated with varying ratios of In/Ga and In/A to
achieve similar graded band gaps, and it could be accomplished by
standard reactive AC sputtering with nitrogen as described in
'010.
The conventional plated CdS n-type window or buffer layer is not
used because of the toxicity and waste disposal problems associated
with cadmium. A substitute material that has been shown to work
about as well is ZnS, as previously noted. This material can
readily be made in the present invention by AC reactive sputtering
from elemental zinc targets used in the setup described in FIG. 11.
In this case the reactive gas that is injected through nozzles 17
is hydrogen sulfide instead of hydrogen selenide. Since hydrogen
sulfide is also a dangerous gas, it is not the method of choice for
depositing the layer. Since the layer is very thin, it can be RF
sputtered without negatively impacting the manufacturing rate.
However, as stated before, conventional RF sputtering presents
challenges in large-scale because of the non-uniformity associated
with variable machine geometry. The method of RF sputtering
described below will overcome the objection presented by variable
geometry. ZnSe could be RF sputtered using the same RF technique,
but it is less desirable than ZnS because its band gap is smaller,
although both materials have larger band gaps than CdS.
Since most transparent conducting oxides are n-type semiconductors,
it is somewhat of a mystery that the conventional ZnO, being an
n-type semiconductor, cannot also be used as the window layer to
make the p-n junction. All previous attempts to do this have failed
to yield a cell with high efficiency unless a plated CdS "buffer"
layer is placed in between the absorber and the ZnO. Some studies
have pointed to the formation of gallium oxide at the interface as
being at least part of the problem, although indium oxide and
selenium oxide could form as well. Oxidation damage to the
interface can be caused by energetic negative oxygen ions from the
sputtering plasma bombarding the CIGS surface during the initial
growth phase of the ZnO overcoat. Also, the energetic ions may
cause physical damage to the interface.
This interface damage to the p-n junction may be minimized or
eliminated with the use of a very thin sacrificial layer of a pure
metal placed over the CIGS layer before the transparent conductive
overcoat is applied. It is well known that zinc, cadmium, and
mercury doping will change CIGS from p to n-type, but only zinc is
substantially free of toxicity and waste disposal problems. If a
thin layer of zinc is used, it can serve a dual role. First, it can
diffuse into the CIGS layer doping it to n-type, and therefore move
the p-n junction away from the interface forming a homojunction.
Secondly, it can "take the brunt" of the negative ion bombardment
converting to ZnO or ZnS in the process, thus reducing or
eliminating damage to the CIGS interface. Damage to the interface
is not necessarily limited to high-energy oxygen ions. Both sulfur
and selenium form high-energy ions in a sputtering plasma in a
manner similar to oxygen. Metals other than zinc might be used;
however, they would form p-n heterojunctions. For example, thin
layers of some of the transition metals would protect the CIGS from
oxidation, but would not move the p-n junction by diffusion into
the CIGS. In particular zirconium will be converted to zirconium
oxide, which is also an n-type semiconductor, and it is one of the
alternative materials mentioned by Ullal, Zweibel, and von
Roedern.
The use of a sacrificial layer as just described can help protect
the p-n junction and maintain a higher voltage across the depletion
region. It is useful because highly conductive ZnO will not support
hole stability as well as a less conductive n-type interface
material. For this reason it has become common practice to use what
is called "intrinsic" ZnO or i-ZnO as an initial thin overcoat for
the CdS to help maintain the depletion zone in regions where the
chemical bath plated CdS is marginal. This form of ZnO is made by
adding more oxygen to the process to make a less conductive and
more transparent form of the material. Of course the use of i-ZnO
alone is damaging to the interface because of the energetic oxygen
ions. Therefore sacrificial metallic layers can substitute for the
conventional plated CdS as long as the oxide that is produced is an
n-type semiconductor.
After the n-type layer is properly formed to create the p-n
junction, the top transparent electrode layer is deposited. A
transparent and conductive form of ZnO has been the conventional
material used for this layer, largely because it is less costly
compared to materials like indium tin oxide (ITO) that is widely
used in the display industry. ZnO is both less conductive and less
thermally stable than ITO; however, ZnO doped with aluminum has the
approximate performance of ITO while retaining much of the cost
advantage of ZnO. The required level of aluminum doping to achieve
this result is about 2 percent. Similar amounts of other dopants
have been shown to work almost as well (see "New n-Type Transparent
Conduction Oxides" by T. Minami in MRS Bulletin, August 2000).
Currently, large scale sputtering of the ITO in the display
industry is accomplished primarily by using planar ceramic targets,
which are conductive but expensive to fabricate. Control of the
reactive process in large scale when metallic targets are used has
met with little success. A similar problem exists with the
large-scale control of the reactive process for depositing aluminum
doped ZnO. In principle this problem can be solved by using ceramic
targets as is done with the ITO, but the extra cost for the target
fabrication offsets much of the advantage gained from the cheaper
materials. In this invention the rotary magnetrons as described in
'010 allow the use of the cheaper metallic targets, and provide the
necessary control of the reactive process for large-scale
implementation. The reactive sputtering setup is identical to that
described in FIG. 11 where targets 8 and 9 are both metallic zinc
targets with the appropriate doping of aluminum. In addition to the
usual argon sputtering gas, oxygen is supplied to the deposition
region through nozzles 17.
FIG. 13 shows the preferred all sputtered CIGS solar cell structure
of the present invention. Layer 1 (the substrate) is a high
temperature metal or polymer foil. Stainless steel, copper, and
aluminum are the preferred metal foils for terrestrial power
production, while very thin titanium and polyimide are preferred
foils for space power applications. Electrically conductive layers
2, 2a, 2b, and 2c are Cu, ZrN, Ag and ZrN, respectively, as
previously described in FIG. 7. The CIGS in layer 3 has a graded
band gap created by changes in the composition of successive
targets as shown and described in FIG. 12. The method of deposition
may be either the DC co-sputtered film described in FIG. 9 or the
reactively sputtered film described in FIG. 11. Semiconductor layer
4 is RF sputtered ZnS (or ZnSe) replacing the CdS of the
conventional cell. As an additional feature, layer 4a may be
included as a sacrificial metal layer that becomes an n-type
semiconductor upon subsequent reaction during the deposition of the
next layer (i.e. with oxygen, sulfur, or selenium). Layer 5 is the
transparent top electrode and comprises of reactively deposited
aluminum doped ZnO to take advantage of the improvement in
performance over the conventional ZnO. As previously explained, a
very thin portion of the aluminum doped ZnO at the layer 4
interface may have a higher resistivity to improve the junction
voltage. Layer 6 is the optional anti-reflection AR) film and is
actually a multi-layer stack (not shown) designed to optimize the
light absorption in the cell. Such an AR stack would be used in a
space power application where environmental degradation from
weather is not an issue. For terrestrial applications the basic
cells (layers 1 through 5) are laminated to a protective glass
cover sheet in a sealed module (not shown), and the AR layer, if
used, is applied to the outer surface of the glass cover instead of
directly to the cell. One might additionally sputter current
collection grid lines on the top conducting oxide, and if the
substrate is a metal foil, a thin solder wetting layer (tin for
example) may be sputtered on the back.
A simplified schematic side view of a roll-to-roll modular
sputtering machine for making the improved solar cell of FIG. 13 is
illustrated in FIG. 14. In the direction perpendicular to the view
plane the machine is sized to support substrates between about two
and four feet wide. This width is not a fundamental equipment
limit; rather, it recognizes the practical difficulty of obtaining
quality substrate material in wider rolls. The machine is equipped
with an input, or load, module 21a and a symmetrical output, or
unload, module 21b. Between the input and output modules are
process modules 22a, 22b, and 22c. The number of process modules
may be varied to match the requirements of the coating that is
being produced. Each module has a means of pumping to provide the
required vacuum and to handle the flow of process gases during the
coating operation. The vacuum pumps are indicated schematically by
elements 23 on the bottom of each module. A real module could have
a number of pumps placed at other locations selected to provide
optimum pumping of process gases. High throughput turbomolecular
pumps are preferred for this application. The modules are connected
together at slit valves 24, which contain very narrow low
conductance isolation slots to prevent process gases from mixing
between modules. These slots may be separately pumped if required
to increase the isolation even further. Alternatively, a single
large chamber may be internally segregated to effectively provide
the module regions, but it then becomes much harder to add a module
at a later time if process evolution requires it.
Each process module is equipped with a rotating coating drum 25 on
which web substrate 26 is supported. Arrayed around each coating
drum is a set of dual cylindrical rotary magnetron housings 27.
Conventional planar magnetrons could be substituted for the dual
cylindrical rotary magnetrons; however, efficiency would be reduced
and the process would not be as stable over long run times. The
coating drum may be sized larger or smaller to accommodate a
different number of magnetrons than the five illustrated in the
drawing. Web substrate 26 is managed throughout the machine by
rollers 28. More guide rollers may be used in a real machine. Those
shown here are the minimum needed to present a coherent explanation
of the process. In an actual machine some rollers are bowed to
spread the web, some move to provide web steering, some provide web
tension feedback to servo controllers, and others are mere idlers
to run the web in desired positions. The input/output spools and
the coating drums are actively driven and controlled by feedback
signals to keep the web in constant tension throughout the machine.
In addition, the input and output modules each contain a web
splicing region 29 where the web can be cut and spliced to a leader
or trailer section to facilitate loading and unloading of the roll.
Heater arrays 30 are placed in locations where necessary to provide
web heating depending upon process requirements. These heaters are
a matrix of high temperature quartz lamps laid out across the width
of the coating drum (and web). Infrared sensors provide a feedback
signal to servo the lamp power and provide uniform heating across
the drum. In addition coating drums 25 are equipped with an
internal controllable flow of water or other fluid to provide web
temperature regulation.
The input module accommodates the web substrate on a large spool
31, which is appropriate for metal foils (stainless steel, copper,
etc.) to prevent the material from taking a set during storage. The
output module contains a similar spool to take up the web. The
pre-cleaned substrate web first passes by heater array 30 in module
21a, which provides at least enough heat to remove surface adsorbed
water. Subsequently, the web can pass over roller 32, which can be
a special roller configured as a cylindrical rotary magnetron. This
allows the surface of electrically conducting (metallic) webs to be
continuously cleaned by DC, AC, or RF sputtering as it passes
around the roller/magnetron. The sputtered web material is caught
on shield 33, which is periodically changed. Another
roller/magnetron may be added (not shown) to clean the back surface
of the web if required. Direct sputter cleaning of a conductive web
will cause the same electrical bias to be present on the web
throughout the machine, which, depending on the particular process
involved, might be undesirable in other sections of the machine.
The biasing can be avoided by sputter cleaning with linear ion guns
instead of magnetrons, or the cleaning could be accomplished in a
separate smaller machine prior to loading into the large roll
coater. Also, a corona glow discharge treatment could be performed
at this position without introducing an electrical bias. If the web
is polyimide material electrical biases are not passed downstream
through the system. However, polyimide contains excessive amounts
of water. For adhesion purposes and to limit the water desorption,
a thin layer of metal (typically chromium or titanium) is routinely
added. This makes the surface conductive with similar issues
encountered with the metallic foil substrates.
Next the web passes into the first process module 22a through valve
24 and the low conductance isolation slots. The coating drum is
maintained at an appropriate process temperature by heater array
30. Following the direction of drum rotation (arrow) the full stack
of reflection layers begins with the first two magnetrons
depositing the base copper layer (2 in FIG. 13). The next magnetron
provides a thin ZrN layer, followed by the thin silver layer and
the final thin ZrN layer. For a CIGS absorber layer the band gap is
low enough that little is gained by the thin silver and final thin
ZrN layer. In this case the reflector may consist of just the base
copper layer and the first ZrN layer. Future higher band gap
materials could benefit from the extra silver and ZrN layers.
The web then passes into the next process module, 22b, for
deposition of the p-type graded CIGS layer. Heater array 30
maintains the drum and web at the required process temperature. The
first magnetron deposits a layer of copper indium diselenide while
the next three magnetrons put down layers with increasing amounts
of gallium (or aluminum), thus increasing and grading the band gap
as previously described. The grading may be inverted by
rearrangement of the same set of magnetrons. The last magnetron in
the module deposits a thin layer of n-type ZnS (or ZnSe) by RF
sputtering from a planar magnetron, or a sacrificial metallic
layer, which becomes part of the top n-type layer and defines the
p-n junction.
Next the web is transferred into the final process module, 22c,
where again heater array 30 maintains the appropriate process
temperature. The first magnetron deposits a thin layer of aluminum
doped ZnO which has a higher resistance to form and maintain the
p-n junction in coordination with the previous layer. The remaining
four magnetrons deposit a relatively thick, highly conductive and
transparent aluminum doped ZnO layer that completes the top
electrode. Extra magnetron stations (not shown) could be added for
sputtering grid lines using an endless belt mask rotating around
the magnetrons. If an AR layer is to be placed on top of the cell,
the machine would have an additional process module(s) in which the
appropriate layer stack would be deposited. The extra modules could
also be equipped with moving, roll compatible, masking templates to
provide a metallic grid and bus bar for making electrical contact
to the top electrode. The extra modules and masking equipment adds
significantly to the cost of producing the cell, and may only be
justified for high value added applications like space power
systems.
Finally, the web passes into output module 21b, where it is wound
onto the take up spool. However, an additional operation can be
performed here, which is beneficial in the later processing of the
cells into modules. A dual cylindrical rotary magnetron 34 becomes
the means to pre-wet the back of the substrate foil with solder.
Metallic tin probably has the best properties of the available
solder materials for use with a stainless steel foil, but there are
many solder formulations that will work. Pre-wetting may be
unnecessary for a copper foil if it is kept clean. An ion gun
sputter pre-cleaning of the back surface of the foil before the
solder sputtering may also be done in the output module similar to
that in the input module. In addition the web temperature must be
below the melting point of the pre-wetting solder (about 232 C. for
tin).
FIG. 15 shows a typical process module with an expanded section
revealing details of coating drum 25 and magnetron housing 27. The
coating drum is constructed with a double wall defining a gap 35
through which a cooling gas or liquid may be circulated to regulate
the temperature of the drum and web 26. The web is maintained in
tight contact against the outer surface of the drum. Magnetron
housing 27 consists of a local rectangular chamber 36 that contains
rotary magnetrons 37 and 38 and the associated mounting hardware.
(not shown). The entire housing can be located at a variable but
uniform distance, represented by gap 39, from the surface of the
coating drum and web. This variable gap allows control of the flow
of the sputtering gases from chamber 36 into the larger process
module 22a, which is vigorously pumped. Thus a large pressure
differential is maintained between the background pressure in
rectangular chamber 36 and the process module (22a), and each
magnetron is effectively isolated from its neighbors. Argon
sputtering gas, indicated by the arrow, is fed into chamber 36
through a set of tubes 40 which are spaced uniformly along its
length. For reactive sputtering, the reactive gas (e.g. oxygen,
nitrogen, hydrogen sulfide, hydrogen selenide, etc.) is fed into
chamber 36 through two sets of tubes 41, each set being equally
spaced along its length. Internal baffles 42 create corridors which
direct the reactive gas to the substrate, yet prevent coating flux
from changing the conductance path of the gas with time, insuring a
steady state process. This setup closely resembles that disclosed
by Chahroudi in U.S. Pat. No. 4,298,444 issued 3 Nov. 1981, and is
incorporated herein by reference. Rectangular chamber 36 has been
referred to as a "mini" chamber within the larger vacuum chamber.
The major improvements are that dual cylindrical rotary magnetrons
are substituted for the single rectangular magnetron of the prior
art, and the method of injection of the sputtering gases has been
improved.
Rectangular "mini" chamber 36 provides the key to the use of RF
sputtering for the deposition of the ZnS (or ZnSe) buffer layer
from a single planar magnetron, as opposed to the rotary magnetrons
that are illustrated. This chamber forms an isolated geometrically
uniform structure that in turn provides a uniform electrical
environment for the RF sputtering. This allows the RF sputtering to
proceed uniformly along the length of the magnetron. In addition,
the chamber is protected from contamination from the other
neighboring sputtering sources, so that minor back sputtering from
the chamber walls consists only of the ZnS material. Thus the ZnS
n-type layer is protected from extraneous doping by foreign
contaminants.
It is to be understood that the present invention is not limited to
the embodiment(s) described above and illustrated herein, but
encompasses any and all variations falling within the scope of the
appended claims. For example, as is apparent from the claims and
specification, not all method steps need be performed in the exact
order illustrated or claimed, but rather in any order that allows
the proper formation of the solar cells of the present
invention.
* * * * *